1. Field of the Invention
The present invention is directed to a device for optically transmitting binary information in a free medium between two or a number of units.
2. Description of the Prior Art
Non-galvanic transmission of information (i.e., transmission without the use of a direct path for electrical current between a transmitter and a receiver) can be employed in a plurality of fields for the transmission of digital information between units. One particular application is for transmissions between a medical implant's extracorporeal programmer and the programmer's base unit, e.g., a printer. Although a number of applications for the invention exist, e.g., in transmitting binary information between a printer and a computer or, generally, in transmitting binary information between mobile units without any galvanic contact between the units, it can be most easily understood and described in conjunction with one such application, i.e., a medical implant programmer, described below.
A programmer is an extracorporeal programming unit employed for programming a medical implant such as a pacemaker or an insulin pump. In the programming of the implant (exemplified below with a pacemaker), the programmer (or more accurately its telemetry coils), is applied to the patient's skin over the implant site in order to communicate with the implant by means of radio signals. Transmitted information can concern, e.g., a change in the pacemaker's stimulation interval or retrieval of ECG signals stored for diagnostic purposes in the pacemaker. The programmer communicates in turn with, e.g., a base unit when the programmer's software is to be updated and/or for downloading acquired information. The base unit comprises, e.g., a printer and a computer. Methods are currently known which make use of the programmer's telemetry coils both for the aforementioned telemetric transmissions to/from an implant and for communications between the programmer and the base unit. The amount of information which has to be transmitted between the programmer and base unit, however, is constantly increasing in step with the increasing complexity of contemporary pacemakers. For a programmer, several megabytes of information may have to be transmitted.
The transmission rate using telemetry coils for transmitting data between the programmer and base unit, however, is limited to a few thousand bits per second because, among other things, the time constants of the telemetry coils are relatively long. This results in unreasonably long transmission times being required when the programmer's telemetry coils have to transmit information in megabyte quantities. A significant increase in the transmission rate would therefore be desirable. Taking the programmer to the physician for pacemaker re-programming must also be easy, and therefore the programmer must be battery-powered. Low energy consumption is therefore an additional desirable feature.
British Specification GB 2 228 595 describes a device for optical transmission of digital information. This known device consists of a remote control unit which transmits signals to a receiver unit by means of, e.g., IR light. The signals are subjected to PPM which means that bursts of pulses are emitted at a specific interval between the first pulse in each pulse burst. The signal is modulated by varying the number of pulses in the burst of pulses.
In the known optical transmission described in this British Specification GB 2 228 595, communications are performed using a specific form of carrier wave modulation. One disadvantage with this kind of transmission art is that it consumes a relatively large mount of energy. Another disadvantage is that the transmission rate is not very fast.
U.S. Pat. No. 4,628,541 discloses an infra-red communications system for transmission of binary signals from a battery powered keyboard to a microcomputer. Each binary signal is transmitted from the keyboard to the microcomputer as a few (e.g., two or three) short infra-red pulses indicating the leading edge of the binary signal. Each binary bit ("1" or "0") to be transmitted is coded so that they can be distinguished from each other and then the infra-red generator is activated for the transmission. Since each bit to be transmitted is activating the generator, the system in U.S. Pat. No. 4,628,541 suffers the same disadvantages as the system of British Specification GB 2 228 595, namely it consumes a relatively large amount of energy and the transmission rate is not very fast.
European Application 0 585 030 discloses an infra-red transmission link between a base section and an external device, e.g. a handset. The bit-stream can be transmitted on the infra-red transmission path using binary amplitude modulation (ASK, Amplitude Shift Keying), in which the relative amplitude of the transmitted carrier is one for a transmitted -1-bit, and zero for a transmitted "0"- bit. The emitting frequency of the IR-LED can be used as the modulating carrier, whereby only the "1"-bits cause infra-red emission when the bit stream is transmitted. One "1"-bit can be transmitted as a short IR-pulse. The transmission system disclosed in European Application 0 585 030 is based on a mutual synchronization between a master-clock in the base section of the system and a clock generator in the handset. This synchronization of the clock generator in the handset is recovered from the transmitted bit-stream and requires special hardware, both in the base section and in the handset.